Recently, as for information instruments, ones equipped with a touch panel display have become mainstream, as seen in tablet PCs, smartphones, e-book readers, etc. A touch panel display has a structure where a touch sensor glass and a cover glass are layered on a glass substrate for display. There is also known one with an integrated configuration of a touch sensor glass and a cover glass, which is called OGS (one glass solution).
In a touch sensor glass, a cover glass and an OGS glass, any glass is desired to be thin and have a high strength, for which a glass that has been chemically strengthened through ion exchange, that is, a chemically strengthened glass is used.
The strength characteristics of these chemically strengthened glasses are generally expressed as a surface compressive stress (CS, compressive stress) and a depth of compressive stress (DOL, depth of layer). In the case where a mother sheet glass of ordinary soda lime silicate glass is subjected to chemical strengthening treatment, in general, a chemically strengthened glass having CS of 500 to 600 MPa and DOL of 6 to 10 μm can be obtained.
There has been proposed an aluminosilicate glass having an easily ion-exchangeable composition for enhancing the strength, and in the case where a mother sheet glass of an aluminosilicate glass is subjected to the same chemical strengthening treatment, there can be obtained a chemically strengthened glass having CS of 700 to 850 MPa and DOL of 20 to 100 μm.
However, since the aluminosilicate glass exhibits a high ion-exchanging rate but has a high viscosity, it is problematic in that the productivity thereof is poor and the production cost thereof is high.
On the other hand, soda lime silicate glass is inexpensive as compared with aluminosilicate glass. However, regarding the chemically strengthened glass of an already-existing soda lime glass, since the ion-exchanging rate thereof is low, CS thereof could hardly be increased to a level of glass strength that has become required recently. Accordingly, there has been proposed a chemical strengthening treatment method that can increase the glass strength in a chemically strengthened glass using a soda lime silicate glass (for example, PTL 1).
Moreover, in the chemical strengthening treatment, in order to perform ion exchange efficiently, it is effective to make MgO contained. MgO is known to have the largest effect of accelerating alkali ion exchange among the components of alkaline earth metal oxides (RO) (for example, PTL 2). Since MgO has an action to raise the liquidus temperature TL, i.e., the devitrification temperature, PTL 2 discloses a glass composition containing SrO for compensating it.